Shape memory polymers

ABSTRACT

New shape memory polymer compositions, methods for synthesizing new shape memory polymers, and apparatus comprising an actuator and a shape memory polymer wherein the shape memory polymer comprises at least a portion of the actuator. A shape memory polymer comprising a polymer composition which physically forms a network structure wherein the polymer composition has shape-memory behavior and can be formed into a permanent primary shape, re-formed into a stable secondary shape, and controllably actuated to recover the permanent primary shape. Polymers have optimal aliphatic network structures due to minimization of dangling chains by using monomers that are symmetrical and that have matching amine and hydroxl groups providing polymers and polymer foams with clarity, tight (narrow temperature range) single transitions, and high shape recovery and recovery force that are especially useful for implanting in the human body.

CROSS-REFERENCE TO RELATED APPLICATIONS

This application is a Divisional of U.S. pending patent application Ser.No. 12/905,949, filed Oct. 15, 2010, entitled “Shape Memory Polymers”,which is a Continuation-in-part of application Ser. No. 11/203,025,filed Aug. 12, 2005, now abandoned, which claims the benefit of U.S.Provisional Patent Application No. 60/602,083 filed by Thomas S. Wilsonand Jane P. Bearinger on Aug. 16, 2004, titled “New Shape MemoryPolymers”; and this application claims the benefit of U.S. ProvisionalPatent Application No. 61/332,039, filed May 6, 2010, titled “ShapeMemory Polymers That Cure Post-Polymerization”, which are incorporatedherein by these references.

STATEMENT AS TO RIGHTS TO INVENTIONS MADE UNDER FEDERALLY SPONSOREDRESEARCH AND DEVELOPMENT

The United States Government has rights in this invention pursuant toContract No. DE-AC52-07NA27344 between the United States Department ofEnergy and Lawrence Livermore National Security, LLC for the operationof Lawrence Livermore National Laboratory.

BACKGROUND

1. Field of Endeavor

The present invention relates to shape memory systems and moreparticularly to shape memory polymers.

2. State of Technology

U.S. Pat. No. 3,624,045 (Stivers) describes the development ofcrosslinked polyurethanes exhibiting such properties.

U.S. Pat. No. 5,049,591 (Hayashi)—The use of the term “shape memorypolymer” appears to start with U.S. Pat. No. 5,049,591 (Hayashi), whodescribes compositions of polyurethanes which could be suitable forthermally insulating foams which utilize the shape memory effect forapplication and transportation advantages.

U.S. Pat. No. 5,330,483 (Heaven) appears the first to use shape memorypolymers in a medical application, using them in a fiber mesh comprisinga tissue isolation bag. They are used for the shape recovery property ofan SMA or SMP mesh to pulverize tissue in the bag.

U.S. Pat. No. 5,603,772 (Phan) describes the use of SMPs inintravascular stents. This patent provides example geometries for astent and provides examples of SMP materials. This patent also describesan endoprosthetic device with therapeutic compound (U.S. Pat. No.5,674,242).

U.S. Pat. No. 5,762,630 (Bley) described an SMP thermally softeningcatheter stylet. This allows the catheter to be stiff (below Tg)external to the vasculature and soft (>Tg) within the vasculature.

U.S. Pat. No. 5,911,737 (Lee) and U.S. Pat. No. 6,059,815 (Lee)describes the use of SMPs for micro-actuators used to controllablyrelease arbitrary objects within vascular passageways. These patentsdescribe heating methods to achieve actuation including optical heating,resistive heating, and convective heating using a heat transfer fluid.U.S. Pat. No. 6,086,599 (Lee) also describes SMP micro-structures usedto form mated connections which can be used to reposition or removedevices from otherwise inaccessible places within the body such as thevasculature.

U.S. Pat. No. 5,957,966 (Schroeppel) describes an implantable tubularsleeve used as a catheter, cardiac stimulator lead, or shunt made out ofSMP. The lead can be positioned within a vascular passageway whilecompliant and allowed to harden, taking on the shape of the vessel.

U.S. Pat. No. 5,964,744 (Balbierz) describes the use of SMPs in devicesfor swellable ureteral stents. The SMP acts to hold a second material ina collapsed state. Upon actuation, which occurs when the SMP swells atthe site of use, the whole structure is able to expand. A large numberof specific polymer systems are described.

U.S. Pat. No. 6,090,072 (Kratoska) describes an SMP expandableintruducer sheath, utilizing the shape memory effect to allow it toexpand in diameter after intruction through a relatively small puncture.The sheath can then be expanded to the needed size during the procedurewith no further trauma to point to insertion.

U.S. Pat. No. 6,102,917 (Maitland) and International Patent No.WO0003643 (Maitland) describe an SMP micro-gripper which is opticallyactuated, and can be used to release devices such as embolic coils attargeted sites within the body. The occurrence of actuation can bedetected via the same optical system used for actuation.

U.S. Pat. No. 6,388,043 (Langer) describes SMP compositions, articles ofmanufacture, methods of preparation and use. Compositions allow forpolymer segments to be linked via functional groups which may be cleavedin response to application of energy. U.S. Pat. No. 6,160,084 (Langer)describes biodegradable shape memory polymer compositions, methods ofmanufacture and preparation, and use.

WO 95/26762, published Oct. 12, 1995, Bruin, et al discloses thermalshape memory biodegradable polymers that are useful for manufacture ofstents. The SMP include amorphous non-crystallizable polylactic acidnetworks, the highly cross-linked networks, that also include theconversion products of star prepolyesters such as those based on lacticacid copolymers with di-isocyanate.

SUMMARY

Features and advantages of the present invention will become apparentfrom the following description. Applicants are providing thisdescription, which includes drawings and examples of specificembodiments, to give a broad representation of the invention. Variouschanges and modifications within the spirit and scope of the inventionwill become apparent to those skilled in the art from this descriptionand by practice of the invention. The scope of the invention is notintended to be limited to the particular forms disclosed and theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides new shape memory polymer compositions,methods for synthesizing new shape memory polymers, and apparatuscomprising an actuator and a shape memory polymer wherein the shapememory polymer comprises at least a portion of the actuator. In oneembodiment, the shape memory polymer (SMP) can be formed into a specific“primary” shape, compressed into a “secondary” stable shape, and thencontrollably actuated so that it recovers its primary shape.

The present invention provides a shape memory polymer comprising apolymer composition which physically forms a network structure whereinthe polymer composition has shape-memory behavior and can be formed intoa permanent primary shape, re-formed into a stable secondary shape, andcontrollably actuated to recover the permanent primary shape.

In one embodiment the polymer composition is a thermoset polymer havinga covalently bonded network structure. In another embodiment the polymercomposition has a composition including monomers with high structuralsymmetry in their molecular structure and the resulting shape memorypolymer has a network structure that is highly regular in at least oneor more of molecular weight between network junction points, compositionbetween network junction points, number of chain atoms between networkjunction points, or size of pores formed by network features. In anotherembodiment the polymer composition is composed of monomers with anaverage functionality between 2.1 and 8.

In one embodiment the present invention provides an apparatus comprisingan actuator and a shape memory polymer operatively connected to theactuator wherein the shape memory polymer has a polymer compositionwhich physically forms a network structure wherein the polymercomposition has shape-memory behavior and can be formed into a permanentprimary shape, re-formed into a stable secondary shape, and controllablyactuated to recover the permanent primary shape

The present invention has many uses. For example, applications for thepresent invention include improved actuators, medical devices forinterventional procedures, components of medical devices forinterventional procedures, micro-pumps and valves for use in MEMSsystems, bioanalytical systems for pathogen detection, improvedmicro-grippers and positioning devices, release devices, components oftoys, optical components such as lenses and waveguides, shape memorypolymer foams, data storage media, adaptive structures for aerospaceapplications, and in space applications where reliable deployment is ascritical as low density constraints, and other uses.

The invention is susceptible to modifications and alternative forms.Specific embodiments are shown by way of example. It is to be understoodthat the invention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

BRIEF DESCRIPTION OF THE DRAWINGS

The accompanying drawings, which are incorporated into and constitute apart of the specification, illustrate specific embodiments of theinvention and, together with the general description of the inventiongiven above, and the detailed description of the specific embodiments,serve to explain the principles of the invention.

FIG. 1 shows a microgripper in position to clamp onto a mass ofmaterial.

FIG. 2 shows a microgripper clamped onto a mass of material.

FIG. 3 shows a valve with the valve actuator element in the closedposition.

FIG. 4 shows a valve with the valve actuator element in the openposition.

FIGS. 5A, 5B, and 5C illustrate another embodiment of the presentinvention.

FIG. 6 is a graph that illustrates the range of rubber plateau modulusof the shape memory polymer compositions of the present invention.

FIG. 7 is a graph that illustrates the dynamic storage modulus (G′) ofpolyurethane SMPs of the present invention

DETAILED DESCRIPTION OF THE INVENTION

Referring to the drawings, to the following detailed description, and toincorporated materials, detailed information about the invention isprovided including the description of specific embodiments. The detaileddescription serves to explain the principles of the invention. Theinvention is susceptible to modifications and alternative forms. Theinvention is not limited to the particular forms disclosed. Theinvention covers all modifications, equivalents, and alternativesfalling within the spirit and scope of the invention as defined by theclaims.

The present invention provides new shape memory polymer compositions,methods for synthesizing new shape memory polymers, and apparatuscomprising an actuator and a shape memory polymer wherein the shapememory polymer comprises at least a portion of the actuator. The presentinvention has many uses. For example, applications for the presentinvention include improved actuators, medical devices for interventionalprocedures, components of medical devices for interventional procedures,micro-pumps and valves for use in MEMS systems, bioanalytical systemsfor pathogen detection, improved micro-grippers and positioning devices,release devices, components of toys, optical components such as lensesand waveguides, space applications where reliable deployment is ascritical as low density constraints, and other uses.

Referring now to the drawing and in particular FIGS. 1 and 2, anembodiment of the present invention is illustrated. This embodiment isincorporated in a microgripper that utilizes a pair of Shape MemoryPolymer (SMP) actuators to open and close the jaws of gripping membersaccording to the temperature the SMP actuators are exposed to. Themicrogripper is designated generally by the reference numeral 10.

The microgripper 10 is composed of a pair of grip arms or grippingmembers 11 and 12 formed, for example, from silicon wafers. Eachgripping member 11 and 12 include a reduced thickness of cross-sectionarea 13 and 14 and gripping jaws 15 and 16. By way of example, the griparms or gripping members 11 and 12 may be constructed of silicon, orcompatible metals, polymers, or ceramics with an overall combined heightand width thereof preferably not to exceed 250 μm, with the thickness ofmembers 11 and 12 being 20 to 100 μm, with reduced areas 13 and 14having a thickness of 5 to 15 μm, and gripping jaws 15 and 16 extendinga distance of 20 to 50 μm. The actuators 18 and 19 are secured togripping members 15 and 16 adjacent the reduced areas 13 and 14. Theactuators 18 and 19 are constructed of SMP or layers as will bedescribed subsequently.

Referring now to FIG. 1, the microgripper 10 is shown in position toclamp onto a mass of material 17. The mass of material may, for examplebe a clot in a vein or artery. The mass of material may be othermaterials that need to be removed from an area where access isdifficult. For example, part of the nation's Stockpile Stewardshipprogram focuses on assessing the condition of weapons and understandingthe effect of aging on them. With a better understanding ofaging—together with the development of new diagnostic tools and improvedanalysis methods—stockpile surveillance can be more predictive, makingit possible to correct developing problems. The mass of material 17could be a material inside of a weapon that needs to be gripped andremoved without disturbing components of the weapon and withoutdisassembling the weapon.

The gripping members 11 and 12 of the microgripper 10 are shown openwith the jaw 15 positioned above the mass 17 and the jaw 16 positionedbelow the mass 17. It will be appreciated that by closing the jaws 15and 16 the mass 17 will be securely gripped by the microgripper 10.

Movement of the gripping members 11 and 12 jaws and the jaws 15 and 16is actuated by the SMP actuators 18 and 19. Each gripping member 11 and12 include a reduced thickness of cross-section area 13 and 14. Theactuators 18 and 19 are secured to gripping members 15 and 16 adjacentthe reduced areas 13 and 14. Upon heating of the actuators 18 and 19 bya heater, not shown, the actuators expand or contract causing outward orinward flexing or bending of the outer gripping jaws 15 and 16 ofgripping members 11 and 12 at reduced thickness areas 13 and 14 causingthe gripping jaws 15 and 16 to separate or retract.

Referring now to FIG. 2, the microgripper 10 is shown with the grippingmembers 11 and 12 jaws and the jaws 15 and 16 closed thereby grippingthe mass of material 17. The gripping members 11 and 12 rotate about thereduced thickness areas 13 and 14. The actuators 18 and 19 supply theforce to expand or contract and cause outward or inward flexing orbending of the gripping members 11 and 12 at reduced thickness areas 13and 14 causing the gripping jaws 15 and 16 to separate or retract. Themass of material 17 is securely gripped by the jaws 15 and 16 when theactuators 18 and 19 have contracted and caused the gripping members 11and 12 to move to the closed position.

The actuators 18 and 19 are constructed of SMP having a thickness of 2to 5 μm and length of 300 μm to 500 μm. Heating of the SMP actuators 18and 19 is accomplished, for example, by integrating polysilicon heatersor direct resistive heaters into the microgripper 10 or by laser heatingthrough optical fibers.

The actuators 18 and 19 are constructed of SMP or layers. Shape-memorymaterials have the useful ability of being formable into a primaryshape, being reformable into a stable secondary shape, and then beingcontrollably actuated to recover their primary shape. Both metal alloysand polymeric materials can have shape memory. In the case of metals,the shape-memory effect arises from thermally induced solid phasetransformations in which the lattice structure of the atoms changes,resulting in macroscopic changes in modulus and dimensions. In the caseof polymeric materials, the primary shape is obtained after processingand fixed by physical structures or chemical crosslinking. The secondaryshape is obtained by deforming the material while is an elastomericstate and that shape is fixed in one of several ways including coolingthe polymer below a crystalline, liquid crystalline, or glass transitiontemperature; by inducing additional covalent or ionic crosslinking, etc.While in the secondary shape some or all of the polymer chains areperturbed from their equilibrium random walk conformation, having acertain degree of bulk orientation. The oriented chains have a certainpotential energy, due to their decreased entropy, which provides thedriving force for the shape recovery. However, they do not spontaneouslyrecover due to either kinetic effects (if below their lower Tg) orphysical restraints (physical or chemical crosslinks). Actuation thenoccurs for the recovery to the primary shape by removing that restraint,e.g., heating the polymer above its glass transition or meltingtemperature, removing ionic or covalent crosslinks, etc. Additionaltypes of actuators similar to the actuators 18 and 19 illustrated inFIGS. 1 and 2 are shown and described in U.S. Pat. No. 5,645,564 issuedJul. 8, 1997 to Milton A. Northrup et al. U.S. Pat. No. 5,645,564 isincorporated herein in its entirety by this reference.

Shape memory polymers (SMPs) have recently been receiving a great dealof interest in the scientific community for their use in applicationsranging from light weight structures in space to micro-actuators in MEMSdevices. These relatively new materials can be formed into a primaryshape, reformed into a stable secondary shape, and then controllablyactuated to recover their primary shape. Such behavior has been reportedin a wide variety of polymers including polyisoprene, segmentedpolyurethanes and their ionomers, copolyesters, ethylene-vinylacetatecopolymers, and styrene-butadiene copolymers.

Referring now to FIGS. 3 and 4, another embodiment of the presentinvention is illustrated. This embodiment is incorporated in a valveused in a Micro-Electro-Mechanical Systems (MEMS). The MEMS device isdesignated generally by the reference numeral 30.

The MEMS device 30 is composed of a substrate 32 made of a suitablematerial. For example, substrate 32 may be silicon wafer. A fluidpassage 34 extends through the substrate. The fluid passage may beopened and closed by the valve actuator element 33. The MEMS device 30provides a valve that blocks the flow of fluid 31 through the fluidpassage 34 or the flow of allows fluid 31 to flow through the fluidpassage 34. The valve actuator element 32 is constructed of SMP.Shape-memory materials have the useful ability of being formable into aprimary shape, being reformable into a stable secondary shape, and thenbeing controllably actuated to recover their primary shape.

Referring now to FIG. 3, the valve 30 is shown with the valve actuatorelement 33 in the closed position wherein the valve actuator element 33blocks the flow of fluid 31 through the fluid passage 34. The valveactuator element 33 extends across the fluid passage 34 and prevents thefluid 31 from moving the fluid passage. The valve actuator element 33 isconstructed of SMP. Shape-memory materials have the useful ability ofbeing formable into a primary shape, being reformable into a stablesecondary shape, and then being controllably actuated to recover theirprimary shape.

Referring now to FIG. 4, the valve 30 is shown with the valve actuatorelement 33 in the open position wherein the valve 30 is open and thefluid 31 can flow through the fluid passage 34. The valve actuatorelement 33 has changed shape and main portion of the valve actuatorelement 33 has moved upward open the fluid passage 34. The valveactuator element 33 is constructed of SMP. Shape-memory materials havethe useful ability of being formable into a primary shape, beingreformable into a stable secondary shape, and then being controllablyactuated to recover their primary shape.

The structure of the valve 30 having been described, the operation ofthe valve 30 will now be considered. The valve 30 can be open as show inFIG. 4 or closed as shown in FIG. 3. This may is accomplished by heatingthe valve actuator element 33. Heating of the valve actuator element 33is accomplished, for example, by integrating polysilicon heaters ordirect resistive heaters into the valve 30 or by heating through anexternal source of heat. Heating of the valve element 30 causes it tochange its shape. The valve actuator element 33 may be constructed in aprimary shape as illustrated in FIG. 3, reformed into a stable secondaryshape as illustrated in FIG. 4, then controllably actuated to recoverits primary shape. Alternatively, the valve actuator element 33 may beconstructed in a primary shape as illustrated in FIG. 4, reformed into astable secondary shape as illustrated in FIG. 3, then controllablyactuated to recover its primary shape.

The valve actuator element 33 is made of a shape memory polymer of thepresent invention. In one embodiment the valve actuator element shapememory polymer comprises a polymer composition which physically forms anetwork structure wherein the polymer composition has shape-memorybehavior and can be formed into a permanent primary shape, re-formedinto a stable secondary shape, and controllably actuated to recover thepermanent primary shape. In one embodiment the polymer composition is athermoset polymer having a covalently bonded network structure. In oneembodiment the polymer composition has a composition including monomerswith high structural symmetry in their molecular structure and theresulting shape memory polymer has a network structure that is highlyregular in at least one or more of molecular weight between networkjunction points, composition between network junction points, number ofchain atoms between network junction points, or size of pores formed bynetwork features.

In one embodiment the polymer composition has a composition includingstar monomers having equivalent arms and with from 2 to 64 arms and theresulting shape memory polymer has a network structure that is highlyregular in at least one or more of molecular weight between networkjunction points, composition between network junction points, number ofchain atoms between network junction points, or size of pores formed bynetwork features. In one embodiment the polymer composition is composedof monomers with an average functionality between 2.1 and 8.

In one embodiment the polymer composition is a thermoset polymer made bythe crosslinking of a linear polymer which has crosslink sites, definedby reactive functional groups, regularly spaced along the polymer chain,giving rise to a polymer network with a high degree of structuralregularity. In one embodiment the polymer composition has a networkstructure such that the molecular weight of material between junctionpoints is on the order of 0.25 to 100 times the monomer molecularweight.

In one embodiment the polymer composition is optically transparent withclarity ranging from tinted appearance to that approaching glass. In oneembodiment the polymer composition is optically transparent with clarityranging from tinted appearance to that approaching glass and the polymercomposition is amorphous and modified with an index matching material.In one embodiment the polymer composition is optically transparent withclarity ranging from tinted appearance to that approaching glass and thepolymer composition is amorphous and modified with a material which hasa phase size small enough that it does not scatter light. In oneembodiment the polymer composition is optically transparent with clarityapproaching that of glass and including at least one dye, which mayabsorb visible or non-visible wavelengths of light, to promoteadsorption of specific wavelengths of light.

In one embodiment the polymer composition has a Young's modulus in therange of 1 to 100 MPa at a temperature above the glass transitiontemperature. In one embodiment the polymer composition has a very narrowactuation transition range due to the highly regular network structure.In one embodiment the polymer composition has a very low mechanical lossto energy storage ratio defined by the rheological quantitytan(delta)=G″/G′ where G″ is the dynamic loss modulus and G′ is thedynamic storage modulus) at temperatures in which it is in theelastomeric state. In one embodiment the energy storage ratio is below0.03. In one embodiment the energy storage ratio is below 0.01.

In one embodiment the polymer composition is a polyurethane shape memorypolymer. In one embodiment the polyurethane shape memory polymer iscomposed of monomers prepared using a di-functional isocyanate and apolyfunctional alcohol, amine, or carboxylic acid in the mole ratio(based on functional group content) of 1 mole of combined of (hydroxy,amine, and carboxylic acid) groups to 0.8 to 1.2 moles of isocyanategroups. The preferred ratio of (combined hydroxy, amine, and carboxylicacid) functionality to isocynate is 1.0:1.0-1.05.

In one embodiment the polymer composition is prepared using combinationsof N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine (HPED),triethanolamine (TEA), butane diol (BD), and hexamethylene diisocynate(HDI), with the following range of compositions based on 1 molesequivalent of HDI: 0.1 to 0.5 moles HPED, 0 to 0.54 moles of TEA, 0 to0.40 moles of BD.

In one embodiment the shape memory polymer includes additives and/orfillers to enhance its physical, mechanical, optical, electrical, ormagnetic properties. In one embodiment the polymer composition includessingle walled or multi-walled carbon nanotubes. In one embodiment thepolymer composition is processed into a closed cell or open cell foamthrough chemical blowing, physical blowing, or porogen templatingtechniques. In one embodiment carbon nanotubes are added to a foamcomposition.

The shape memory polymer of the present invention is produced by methodsthat include a number of steps. In one embodiment the shape memorypolymer of the present invention produces a polymer composition whichphysically forms a network structure wherein the polymer compositionphysically forms a network structure. Methods of making shape memorypolymers are disclosed in U.S. Pat. No. 6,388,043 to Robert Langer andAndreas Lendlein issued May 14, 2002. U.S. Pat. No. 6,388,043 to RobertLanger and Andreas Lendlein issued May 14, 2002 is incorporated hereinby reference.

Referring now to FIG. 5A another embodiment of the present invention isillustrated. This embodiment is incorporated in a microvalve actuator.The microvalve actuator is designated generally by the reference numeral50. The bistable microvalve actuator 50 can best be described as lookinglike a tiny sombrero with a hole in the middle (or one half of a hollowtorus). The microvalve actuator 50 is composed of a Shape Memory Polymer(SMP). Shape-memory materials have the useful ability of being formableinto a primary shape, being reformable into a stable secondary shape,and then being controllably actuated to recover their primary shape.

The microvalve actuator 50 is a bistable microvalve actuator used with amicrocatheter (not shown). In operation, the valve 50 is attached to theend of a microcatheter. The microcatheter with the microvalve actuator50 can be used for controlled release of a substance into a desiredlocation or the collection of a sample from a desired location. When themicrovalve actuator 50 is open and the pressure is positive inside thecatheter, fluid flows out of the catheter through the microvalveactuator 50. When the microvalve actuator 50 is open and the pressure isnegative inside the catheter, fluid can flow into the catheter.

Referring now to FIGS. 5A, 5B, and 5C, the microvalve actuator 50 has anouter wall 53 that forms a ridge 51. The microvalve actuator 50 also hasan inner wall 52 adjacent hole 54. When the valve 50 is open and thepressure is positive inside the catheter, fluid can flow out of thecatheter through the valve 50. When the valve is open and the pressureis negative inside the catheter, fluid can flow into the catheter. Whenpressure is lowered inside the catheter, fluid flows into the catheterthrough the valve 50. To actuate the microvalve actuator 50, the SMP isheated. The valve 50 is closed by heating the inner wall 52. The heatingcan be accomplished using any prior art heating system.

As shown in FIG. 5C, the polymer in its low temperature state. The valve50 is stable in its closed position.

As illustrated in FIG. 5B, the valve 50 is heated and the pressureinside the catheter is lowered. This forces fluid through the valve port54, pulling it open. The valve 10 is locked into its open position. Thevalve actuator 50 can be fabricated using microfabrication techniques onglass and silicon wafers to make a mold cavity or precision machiningtechniques to make micromolds, into which SMP is molded and cured. Usingthese methods, valve actuators 50 can be made of varying thickness downto the order of 10 microns and varying diameter to fit onto the end of acatheter.

This type of catheters is used in many medical applications forminimally invasive surgery. They can be inserted into arteries or veinsand snaked around within the body until their tip is in a desiredlocation, at which point one of several things could happen. Thisincludes imaging through endoscopic fibers, sensing of changes in tipconditions, or controlled release of a substance into the bloodstream.In each of these cases, the tip of the catheter tube must be closed offuntil it is in position, at which point it can be readily opened andclosed. The microvalve actuator 50 provides the opening and closing ofthe catheter tip using heat from a fiber optic laser in the catheter andpressure from pressurized fluid, i.e., saline in the catheter.

The microvalve actuator 50 employs the properties of shape memorypolymer (SMP) for actuation. This material is stiff at ambienttemperature and soft when heated above its glass transition temperatureTg. The material experiences a modulus change of two orders ofmagnitude, which allows it to hold a shape while cold and deform easilywhile hot. The SMP also has the shape memorizing property of being ableto return to its originally molded shape upon heating in the absence ofexternal loads.

Additional types of actuators similar to the valve actuator 50illustrated in FIGS. 5A, 5B, and 5C are shown and described in U.S. Pat.No. 6,565,526 issued May 20, 2003 to Kirk Patrick Seward. U.S. Pat. No.6,565,526 is incorporated herein in its entirety by this reference.

The present invention provides actuators made at least in part of shapememory polymer, shape memory polymer compositions, and methods ofproducing the shape memory polymer. Embodiments of the present inventionare illustrated in FIGS. 1, 2, 3, 4, 5A, 5B, 5B, and 5C. The new SMPcompositions have unique mechanical properties, bridging the gap betweencurrent shape memory polymers and (metal) shape memory alloys.

Referring now to FIG. 6, a graph illustrates the effect of temperatureon elastic modulus for SMAs and SMPs. The range of rubber plateaumodulus of the shape memory polymer compositions of the presentinvention is designated by the shaded area.

The shape memory polymer compositions of the present invention aresynthesized to have greatly increased rubber plateau Young's modulusover current SMPs. Embodiments of the shape memory polymer compositionsof the present invention are network polymers with a network structuresuch that the molecular weight of material between junction points is onthe order of 1 to 100 times the monomer molecular weight. Embodiments ofthe shape memory polymer compositions of the present invention havesimultaneously superior clarity and higher Young's modulus in therubbery state than currently known SMPs, making them more suitable formany medical and non-medical applications than current SMPs.

Examples of Shape Memory Polymer Compositions of the Present Invention

The present invention provides actuators made at least in part of shapememory polymer, shape memory polymer compositions, and methods ofproducing the shape memory polymer. Examples of various embodiments ofthe present invention are described below. The various embodiments ofthe present invention have one or more of the pertinent aspects andcharacteristics described in the examples.

Polyurethane shape memory polymer compositions of the present invention.Samples of polyurethane shape memory polymer compositions of the presentInvention were prepared as follows: All chemicals were obtained fromSigma-Aldrich and used with no additional purifications. Into 20 mlvials combinations of HPED, TEA, and HDI were added such that the numberof moles of hydroxyl groups given by the HPED and TEA equaled the numberof moles of isocyanate groups for 3 grams of HDI. These mixtures werestirred for 1 minute by hand or vortex mixer, degassed for 10 minutesunder vacuum, then loaded into 1 ml polypropylene syringes. Thesesyringes were cured for 1 hour at room temperature, followed by a rampat 0.5° C./minute to 150° C., then held at 150° C. for 1 hr. Sampleswere cooled and solid polymer rods were removed from the syringes. Thesepolymer rods were 4.65 mm in diameter and 60 mm long and opticallyclear. The composition of a few example polymers and their glasstransitions (based on dynamic loss modulus peak) are given in Table 1below. Glass transition values in the table were obtained bydifferential scanning calorimetry (DSC) using the half-height method.The dynamic storage modulus of the composition of the example polymersis plotted versus temperature in FIG. 7. FIG. 7 shows dynamic storagemodulus (G′) of polyurethane SMPs of the present invention. Linearviscoelastic measurements were made using an ARES LS-2 rheometer at afrequency of 1 Hz, variable strain ranging from 0.01 to 1% to maintain“linear viscoelastic behavior” and ramping the temperature at 1°C./minute.

TABLE 1 Example polyurethane SMP compositions and resulting glasstransition temperatures Sample Wt % HDI Wt % HPED Wt % TEA Tg (° C.) 153.9 46.1 0 81 2 55.9 37.5 6.7 70 3 57.1 29.2 13.7 61 4 59.1 20.0 20.955 5 61.0 10.8 28.2 45

(2) Amorphous, single polymer phase networks with the characteristic ofhaving a moderate to high crosslink density and having a moderate tovery low molecular weight between network junctions. The shape memorypolymer compositions are amorphous, single polymer phase networks withthe characteristic of having a moderate to high crosslink density, orequivalently, of having a moderate to very low molecular weight betweennetwork junctions.

Generally, the higher the number of network junctions (crosslinks), thehigher the Young's modulus of the material above it's glass transitiontemperature. The molecular weight of material between junction points ison the order of 0.25 to 100 times the monomer molecular weight.

(3) Optically Transparent Examples. The shape memory polymercompositions are generally optically transparent with clarityapproaching that of glass. The coloration ranges from clear to having ayellowish tint. This aspect of the shape memory polymer makes itespecially useful for applications involving optics such as waveguidesand lenses. Example applications include optically actuating these shapememory polymers in therapeutic devices for removing blood clots, instents, in minimally invasive surgical tools, and in aneurysm occlusiondevices. Dyes may be added to these materials to promote adsorption ofspecific wavelengths of light, for example indocyanine green andplatinum based dyes.

Monomers having a high degree of molecular symmetry. The shape memorypolymer compositions may be made from monomers which have a high degreeof molecular symmetry. This characteristic, coupled with a very lowmolecular weight between crosslinks, gives rise to a polymer networkwith a limited number of modes of relaxation. The resulting polymer willhave a very sharp glass transition dispersion (narrow glass transitionrange) and hence allows for actuation of the SMP with a minimal changein temperature. An example polymer with this property is a polyurethanebased on 2 moles hexamethylene diisocynate (HDI) and 1 moleN,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine (HPED). The actuationof these polymers should require less energy input and a lowertemperature rise than for existing commercial SMPs. Even in the case ofexisting semi-crystalline SMPs whose actuation is based on crystallinemelting which provides for sharp actuation (small change intemperature), the new materials would require less energy and areoptically clear, which a semi-crystalline polymer would not be.Additional suitable monomers include but are not limited to (i)diisocyanates such as methylene diisocyanate, ethylene diisocyanate,propylene diisocyanate, butylene diisocyanate, pentamethylenediisocyanate, heptamethylene diisocyanate, octamethylene diisocyanate,and homologous n-alkane diisocynates,dicyclohexylmethane-4,4′-diisocyanate, methylene biscyclohexanediisocyanate; additional alcohols, amines, and acids include but are notlimited to N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine (HPED),glycerine, erythritol, star oligo(ethylene glycols) trimethylol propane.

The preferred shape memory polymers of some embodiments of thisinvention are especially suitable for use in the human body in formsthat will remain in the body. These polymers have very sharp singleglass transition temperatures to allow changing from one shape toanother. These polymers have a network where the network chainsconnecting crosslink sites are as similar as possible in molecularweight and structure. Dangling chains are minimized by using monomersthat are symmetrical and that have matching amine and hydroxl groups soas to minimize dangling chains in the finished polymer. The polymers arebased on monomers with very high structural symmetry which areincorporated into the network by end-linking. Aliphatic diisocyanates,as opposed to aromatic, polyurethanes are preferred because of theirincreased biocompatibility upon biodegradation.

By the term symmetric monomers is meant monomers whose structureprovides for those chains between crosslinks to have a structure that issymmetric from the center of the chain link. To achieve this, themonomers must have structures that are similarly symmetric. In the caseof the diisocyanate component, 1,6-diisocyanatohexane is a good example.

Thus for diisocyanate monomers, the following monomers which havesimilar symmetry: 1,2-diisocyanatoethane, 1,3-diisocyanatopropane,1,4-diisocyanatobutane, 1,5-diiso-cyanatopentane,1,6-diisocyanatohexane, 1,7-diisocyanatoheptane, 1,8-diisocyanatooctane,1,9-diisocyanatononane, 1,10-diisocyanatodecane, and also1,4-diisocyanatocyclohexane, Methylene-bis(4-cyclohexylisocyanate). Forthe hydroxyl containing monomers, these will have functionalitygenerally of 2 or greater. If the functionality is greater than two,then the arms from the center of the molecule should have as symmetric astructure as possible.

Examples of this are N,N,N′,N′-Tetrakis(2-hydroxypropyl) ethylenediamine(HPED), triethanol amine (TEA), 1,4-butanediol (BD),N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine (HEED),trimethylolpropane ethoxylate, glycerol, polycaprolactone triol),penta-erythritol, 1,6-hexane diol, hexamethylolmelamine and ethyleneglycol. An important aspect of the shape memory polymers of the presentinvention is their biocompatibility since they will be used in the body.It is known that urethane polymers made from aliphatic monomersdecompose in the body into fragments whose toxicity is lower than thosemade from aromatic monomers.

The decomposition of the urethane bond itself results in the urethanenitrogen becoming a primary amine. The result for aromatic isocyanatemonomers is an aromatic diamine, which has specific toxicity to theliver and other organs. Aliphatic diamines derived from decomposition ofaliphatic diisocyanates are much less toxic and can be similar in factto amino acids.

Biostability is also an important aspect of these polymers since they insome embodiment are used as permanent implants in the human body.Biostability is achieved by elimination of ester and/or ether linkagesfrom the structure. Ester linkages in typical urethanes are due to poly(ester) repeat units typically comprising the soft segments/soft phaseof segmented polyurethanes. These are relatively quickly hydrolyzed inthe body versus the urethane group, by a couple of orders of magnitudein rate. So lack of these groups provides better biostability. In thecase of ether linkages, they can be further oxidized and scissioned byenzymatic mechanisms. While this is slower than the ester hydrolysis, itis still faster than the hydrolysis of the urethane groups. Therefore,in one embodiment monomers of symmetric hydroxyl containing monomers arechosen that have no ester or ether linkages.

Another interesting and unpredictable aspect of the polymers made withsymmetric monomers as discussed above is that they have been found tohave few or no dangling chains—the absence of which helps to achieve avery narrow single glass transition temperature range. In order tominimize dangling chains it is it is important to note that the ratio ofmonomers is chosen so as to minimize the dangling chain ends. Forexample, ideally monomers are chosen so each hydroxy group on the polyolhas a matching isocyanate group to react with.

As used herein a narrow range of transition temperature, Tg, meanspolymers having very little structural heterogeneity present in thechains between entanglements or crosslinked for a thermoset system. Asingle sharp transition means the polymer has one glass transition(single tan (delta) peak at Tg or DSC endotherm step) and again, thattransition occurs over a very narrow range of temperatures. The glasstransition in polymers is not a sharp transition like a crystallizationor melting point but occurs over a range of temperature. In a singlephase system, this temperature range covers from the point at which theelastic modulus falls below about 1 GPa, to the high end of the rangewhere the modulus again starts to level off in the beginning of therubber plateau. The reason for this range is that at the onset of Tgthere begins to be segmental mobility in the polymer chainscharacteristic of cooperative motion in just a few consecutive bonds,while as the temperature increases the number of consecutive bonds forwhich there is rotational mobility increases. At the high end of theglass transition segmental motion characteristic of who segments betweenchain entanglements is possible. The breadth of the glass transition(sometimes referred to as the glass transition dispersion) relates tothe structural homogeneity in bonds along the chains, which in turndictates over what range of temperature the thermal energy in thematerial is sufficient to provide for bond rotation/motion.

While it is known, in general, to produce shape memory polymers from thekinds of monomers discussed herein there has been no appreciation of thecombination of unique and unpredictable properties that have beenachieved by the reaction of symmetric aliphatic polymer having little orno dangling chains.

In making crosslinked polymers, because the stoichiometry is imperfectbecause of impurities and molecular mobility, etc. the reacting monomersdo not generally completely react so that a certain amount of monomerfunctional groups remain unreacted. This leaves material which is notpart of what is referred to as network chains, i.e. material which isnot bound on each side to a crosslink site. These groups or in somecases larger chains which have one free end and one end connected to thenetwork are referred to as dangling chains or dangling ends. Such groupsprovide increased heterogeneity to the system and hence a broader Tg aswell as decreased elasticity. By minimizing dangling chain ends ordangling chains, one obtains the sharpest Tg and highest mechanicalproperties (elastic modulus, strength, recoverable strain) possible.

Polymers from these monomer were made as described above and werecharacterized by solvent extraction, NMR, XPS, UV/VIS, DSC, DMTA, andtensile testing to confirm their structure, thermal properties, andmechanical properties relevant to their use as shape memory materials.

Using FIG. 7 as an example, it can be seen that at low temperature thepolymers have moduli in the range of glassy polymers, at highertemperatures the modulus goes through a sharp transition, dropping abouttwo orders in magnitude over about 20° C. temperature span. At thehighest temperatures shown the modulus is in the range of fairly stiffelastomers and increases slightly with temperature. Analysis of plots oftan d v. temperature likewise indicates a single sharp transition withvalues that rapidly go below the instrument resolution above thematerial Tg. The very low values of tan d in the region of rubberybehavior likewise indicate a very low amount of dangling chains and/ordangling groups with very low molecular weight such that theircontribution to internal friction in the material during deformation isnegligible compared to the elastic response in the material.

Monomers which maximize the amount of material comprising the networkbackbone and minimize material making up dangling chain ends. Anotheraspect of the shape memory polymer compositions include a polymernetwork structure wherein monomers are used which maximize the amount ofmaterial comprising the network backbone and minimizing material makingup dangling chain ends.

Glass transition temperature controlled through their composition. Theglass transition temperature of the shape memory polymer compositionsinclude can be controlled through their composition. Currently,materials have been made with glass transitions from 35 to 132° C.Examples of various formulations covering this range of temperature aregiven later.

Additional increases in the Young's modulus of the shape memory polymercompositions include both above and below the Tg can be accomplished bythe addition of carbon nanotubes (single walled, multi-walled, surfacefunctionalized and non-functionalized), exfoliated clay, particulatesilica, and traditional modifiers such as glass fibers, carbon fibers,mineral fillers, metal fillers, glassy polymers, liquid crystallinepolymers. In particular, glassy polymers with matching refractive indexcan be added to both increase modulus and maintain transparency. Also,the additives listed above may also be used to increase the toughness ofthe SMP, thus increasing its ability to store elastic energy.

Carbon nanotubes. Qualities that the carbon nanotubes may impart to theshape memory polymer compositions include adsorption of light or otherelectromagnetic radiation, electrical conductance, thermal conductance,and the use of the carbon nanotubes to enable resistive heating of theSMP when used as a dispersed additive or as a composite device.Additionally, the carbon nanotubes may be used to modify the rheologicalbehavior of the SMP relevant to the processing of the SMP prepolymer orSMP polymer. The structure of the carbon nanotubes within the SMP may beas a random dispersion or the carbon nanotube modified SMP may beprocessed to obtain a specific degree of orientation of the highlyanisotropic carbon nanotubes. For example, when used in the a SMP strandit may be advantageous to maximize the degree of axial orientation ofthe carbon nanotubes in order to obtain a higher modulus and strengththan would be obtained if the carbon nanotubes had a randomconformation. Another example is to promote within plane orientation ofthe carbon nanotubes in an SMP sheet.

Additives to SMP made into a foam. Another specific example of the useof carbon nanotubes in which they would have a number of beneficialeffects would be as additives to SMP that is being made into a foam,which may be open or closed cell in structure. During the foam expansionprocesses the SMP polymer with the nanotubes is subjected todeformations which can be highly extensional in nature. Such extensionflows are expected to significantly orient the carbon nanotubes. Assuch, the carbon nanotubes could act as rheology/flow modifiers, foamstabilizers, to enhance the modulus and toughness of the final foam, toincrease the degree of openess in the foam by acting to destabilize cellmembranes, and to decrease the density of the foam by helping toincrease foam expansion. The carbon nanotubes would also convey to theSMP resin the same properties as described above. As an example of thisembodiment, SMP foams were made in the following way. First, aprepolymer of the SMP was made by mixing 30.1 grams of HDI, 6.31 gramsHPED, and 1.08 grams TEA. This prepolymer was cured for 2 hours at 80°C. To 32.0 grams of the prepolymer was added 2.0 grams of a polyolmixture (85 wt % HPED, 15 wt % TEA), 1.8 grams of Dabco® DC-5169surfactant (Air Products), 1.8 grams Dabco® DC-4000 surfactant (AirProducts), 1.1 grams of water containing 1 wt % SWNT (Carbon SolutionsSWNT-P3, made by addition of SWNT to water followed by 120 minutessonication), 3 ml of Enovate 3000 (Honeywell) as a physical foamingagent, and finally 374 microliters of a catalyst mixture containing 2.5grams Dabco BL-22 and 1.0 grams Dabco T-131 (Air Products). Allcomponents were pre-mixed together by hand for 2 minutes, except for thecatalyst, which was added in last. The catalyst mixture was hand mixedfor 10 seconds and the foam formulation was placed in an oven at 80° C.for 1 hour, followed by a room temperature cure for 24 hours. Theresulting foam had an average density of 0.022 grams/cc.

As further examples, a series of open cell foams were made and tested.Referring to Table 2 for components and addition order the foams weremade from prepolymers in the following manner.

TABLE 2 Typical foam formulations (Target Tg = 45° C.) Addition FoamChemical Purpose Order Weight (grams) HPED foam component 4 2.0Prepolymer foam component 1 32.0 Dabco DC5169 surfactant 2 1.8 DabcoDC4000 surfactant 3 1.8 Dabco T-131 chain catalyst 7 0.107 Dabco BL-22general catalyst 7 0.267 Water chemical blowing agent 5 1.1 Enovate 3000Physical blowing agent 6 3.0 milliliters

Prepolymers were prepared (under nitrogen) by adding the requiredpolyols to the disocyanate monomer, mixing well by vigorous agitationfor 5 minutes followed by room temperature cure for one hour and a slowramp to 50° C. over one additional hour and then curing for 20 hours at50° C. under nitrogen. The resulting prepolymers syrup had a viscosityin the range of about 1000 centipoises.

The components are added in the addition order (1-5) of Table 2 in a 1liter propylene beaker. This is mixed for one minute vigorously by hand.Enovate and catalyst are then added and mixed vigorously for about 15seconds. The mixture is then placed in as n oven at 90° C. (undernitrogen blanket) for 20 minutes during which time foaming takes place.The foam rises from a volume of about 35 ml to as much as 1.5 litersduring this time. The foam is then removed from the oven and allowed tocure at room temperature for an additional 24 hours under inertatmosphere. The foams made in this manner will have 5 to 10% un-reactedisocyanate groups that can be consumed by reaction with moisture or canbe specifically eliminated be reaction with additional monomers (e.g.ethylene glycol) in solvent (e.g. THF or dioxane.) The foam surface canbe modified for controlled biological interaction. The foams may alsocontain significant residual surfactant and catalyst that can be removedby a combination of solvent extraction (THF, dioxane, acetone,isopropanol water sequence) and/or by vacuum drying (90° C., 50 millitorfor 24 hours).

For these foam formulations: Enovate® 3000 (HFC-245fa,1,1,1,3,3,-pentafluoropropane) is a liquid hydrofluorocarbon blowingagent, which was developed as a blowing agent for rigid insulatingfoams. Dabco DC5169 is a silicone emulsifier or surfactant often used inTDI, MDI/TDI and MDI based cold cure flexible molded polyurethane foamsystems. It offers foam stabilization. Possesses very fine foam cellstructure. Provides foam surface appearance. Dabco DC 4000 is a siliconesurfactant. Dabco T-131 is a high-activity organotin gelationcrosslinking catalyst. Dabco BL-22 is a blowing catalyst that promotesopen cells in flexible molded foams and improves friability inwater-blown rigid foams.

SMP polymers based on polyurethane chemistry. Specifically, polymerswhich satisfy the characteristics above can be made which are generallycomposed of a bi-functional isocyanate and a combination of 1 or moremulti-functional hydroxy or amine containing monomers. The hydroxy oramine containing monomers functionality, as defined by Carothers to meanthe number of possible linkages of a monomer molecule, will generally be2 or higher. For example, a functionality of 2 would result in a linearpolymer. Preferably, the average functionality of the hydroxy or aminecontaining monomer would be 3 or higher. Also, it is preferable that thefunctional groups on all monomers be located at the ends of themolecules so as to maximize the percentage of material in the finalpolymer network.

Suitable diisocyanate monomers for the SMPs include but are not limitedto aliphatic diisocyanates such as octamethylene diisocyanate,hexamethylene diisocyanate (HDI), pentamethylene diisocyanate,tetramethylene diisocyanate, trimethylene diisocyanate, ethylenediisocynate, methylene diisocyante, anddicyclohexylmethane-4,4′-diisocyanate; and aromatic diisocyanates suchas methylenedi-p-phenyl diisocyanate (MDI) and toluene diisocyanate(TDI).

Suitable multi-functional hydroxy-containing monomers include but arenot limited to N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine(HPED), triethanol amine (TEA), 1,4-butanediol (BD),N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine (HEED),trimethylolpropane ethoxylate, glycerol, polycaprolactone triol),penta-erythritol, 1,6-hexane diol, hexamethylolmelamine and ethyleneglycol.

Monomers that enhance biocompatibility. The new polyurethane SMPs ofthis invention may be made from monomers so as to enhance theirbiocompatibility. These materials can be made of monomers that allow forcontrollable biodegradation or bio-adsorption. Additionally, the use ofaliphatic monomers will reduce their toxicity upon bio-degradation.

The new SMPs may be based on acrylic monomers. Copolymers are typicallymade so as to have a statistical molecular or network structure based ontheir thermally or photochemically initiated free radicalpolymerization. Such a structure gives rise to a broad glass transitiondispersion, which broadens as the degree of cross-linking increases. Newacrylic SMPs can be made with a highly controlled structure so as toform networks. This is accomplished by controlled radical, anionic, orcationic polymerization schemes so that crosslinkable monomers can beincorporated into the polymer structure in a regular fashion.

Acrylic based SMPs. SMPs consist of a combination of one or moredi-functional monomers as well as a monomer containing a crosslinkablegroup. Suitable di-functional acrylic monomers include but are notlimited to methyl methacrylate, ethyl methacrylate, propyl methacrylate,butyl methacrylate, hexyl methacrylate, methyl acrylate, ethyl acrylate,propyl acrylate, butyl acrylate, acrylic acid, methacrylic acid, andacrylamide.

Suitable cross-linkable monomers include but are not limited to allyltrichlorosilane and allyltriacetoxysilane.

Glass transitions of the new SMPs is adjusted through the use ofplasticizers and anti-plasticizers.

Suitable methods for actuating these new SMPs in their applications areprimary through heating including but not limited to optical heating(e.g., with a laser), resistive heating, inductive heating (e.g., withferro-magnetic particles), acoustic, and through contact with heatedmaterials, e.g., through contact with heated fluids or a heated coating.Additionally these materials may be actuated without change intemperature but by exposure to a changing chemical environment, e.g.,immersion in a plasticizing chemical which shifts the Tg to a lowertemperature.

Shape memory polymers of the present invention have a large number ofpotential uses. These include their use in biomedical applications(interventional devices such as stents, catheters, thrombus removaldevices, vascular filters, sutures, clamps, patches, vascular occlusiondevices, cochlear implants, retinal prostheses), in biodetection systems(valves and other active or passive elements), in bioreactors, inextracorporeal artificial organs, in toys, recreational equipment,utensils, tools, automotive, aerospace (bulk polymer and foams forexpandable extra-terrestrial devices, components, vehicles,instruments), and in controllers. The uses also include uses thatrespond to issues of mechanical modulus, strength, optical clarity,radiological contrast, processibility, biocompatibility, and promotionof specific biological responses. The shape memory polymer may bebiodegradable. Some examples of uses of the present invention includeuse for medical devices for interventional procedures, components ofmedical devices for interventional procedures, micro-pumps and valvesfor use in MEMS systems, bioanalytical systems for pathogen detection,improved micro-grippers and positioning devices, release devices,components of toys, optical components such as lenses and waveguides,space applications where reliable deployment is as critical as lowdensity constraints, and other uses.

While the invention may be susceptible to various modifications andalternative forms, specific embodiments have been shown by way ofexample in the drawings and have been described in detail herein.However, it should be understood that the invention is not intended tobe limited to the particular forms disclosed. Rather, the invention isto cover all modifications, equivalents, and alternatives falling withinthe spirit and scope of the invention as defined by the followingappended claims.

The invention claimed is:
 1. An open cell shape memory polymer foam madefrom a cross-linked polyurethane polymer comprising the reaction productof a symmetric diisocyanate monomer or monomers and a symmetric hydroxylcontaining monomer or monomers.
 2. The polymer foam of claim 1 whereinthe diisocyanate monomer or monomers are aliphatic.
 3. The polymer foamof claim 1 wherein the monomer(s) are aliphatic and wherein themonomer(s) are chosen to provide matching hydroxyl and amine groups inorder to minimize dangling chains in the finished polymer.
 4. Thepolymer of claim 1 wherein the diisocyanate monomer(s) are selected fromthe group consisting of octamethylene diisocyanate, hexamethylenediisocyanate (HDI), pentamethylene diisocyanate, tetramethylenediisocyanate, trimethylene diisocyanate, ethylene diisocyanate,methylene diisocyanate, and dicyclohexylmethane-4,4′-diisocyanate; andaromatic diisocyanates are selected from the group consisting ofmethylenedi-p-phenyl diisocyanate (MDI) and toluene diisocyanate (TDI)and the hydroxyl containing monomers are selected from the groupconsisting of N,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine(HPED), triethanol amine (TEA), 1,4-butanediol (BD),N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine (HEED),trimethylolpropane ethoxylate, glycerol, polycaprolactone triol),penta-erythritol, 1,6-hexane diol, hexamethylolmelamine and ethyleneglycol.
 5. The polymer foam of claim 1 wherein the symmetricdiisocyanate monomer(s) and hydroxyl containing monomer(s) areformulated to a 1:1 ratio of isocyanate to hydroxyl functional groups.6. The polymer foam of claim 1 wherein the diisocyanate monomer ishexamethylene diisocyanate and the hydroxyl containing monomer isN,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine.
 7. The polymer foamof claim 6 wherein the mole ratio of hexamethylene diisocyanate toN,N,N′,N′-Tetrakis (2-hydroxypropyl)ethylenediamine is 2 to
 1. 8. Thepolymer foam of claim 1 wherein the diisocyanate monomer(s) arealiphatic and contain no ether or ester linkages.
 9. The polymer foam ofclaim 1 wherein the polymer foam is dyed to promote adsorption ofspecific wavelengths of light.
 10. The polymer foam of claim 9 whereinthe polymer foam is dyed with indocyanine green or platinum based dyes.11. The polymer foam of claim 1 also comprising additives selected fromthe group consisting of carbon nanotubes, exfoliated clay, particulatesilica, glass fibers, carbon fibers, mineral fillers, metal fillers,glassy polymers and liquid crystalline polymers added to the polymerprior to foaming.
 12. The polymer foam of claim 11 wherein the additiveis glassy polymers with matching refractive index.
 13. The polymer foamof claim 11 wherein the additive is single walled carbon nanotubes. 14.A method of making polymer foams comprising: preparing a pre-polymer byreacting a symmetric diisocyanate monomer with a symmetric hydroxylcontaining monomer; heating and curing the resulting pre-polymer; addingsurfactant(s), chemical blowing agent(s); adding physical agent andcatalyst; and heating the mixture in an inert atmosphere to causefoaming; and recovering the resulting foam.
 15. The method of claim 14wherein the foam is finished by reacting un-reacted isocyanate groupswith additional hydroxyl containing monomer in a suitable solvent. 16.The method of claim 14 wherein residual surfactant remaining on the foamis reduced by a combination of solvent extraction and vacuum drying. 17.The method of claim 14 wherein the pre-polymer diisocyanate monomers areselected from the group consisting of octamethylene diisocyanate,hexamethylene diisocyanate (HDI), pentamethylene diisocyanate,tetramethylene diisocyanate, trimethylene diisocyanate, ethylenediisocyanate, methylene diisocyanate, anddicyclohexylmethane-4,4′-diisocyanate; and aromatic diisocyanatesselected from the group consisting of methylenedi-p-phenyl diisocyanate(MDI) and toluene diisocyanate (TDI) and the hydroxyl containingmonomers are selected from the group consisting of N,N,N′,N′-Tetrakis(2-hydroxypropyl)ethylenediamine (HPED), triethanol amine (TEA),1,4-butanediol (BD), N,N,N′,N′-Tetrakis(2-hydroxyethyl)ethylenediamine(HEED), trimethylolpropane ethoxylate, glycerol, polycaprolactonetriol), penta-erythritol, 1,6-hexane diol, hexamethylolmelamine andethylene glycol.
 18. The method of claim 14 comprising additivesselected from the group consisting of carbon nanotubes, exfoliated clay,particulate silica, glass fibers, carbon fibers, mineral fillers, metalfillers, glassy polymers and liquid crystalline polymers added to thepolymer prior to foaming.